This is an area of maths vocabulary that children meet very early in their lives. However children cannot learn the meanings of words in isolation and parents need to question them and use the vocabulary to help with their understanding. Talking during a trip to a supermarket could involve using most of these words:
compare
double
half, halve
pair
count out, share out
left, left over
money
coin
penny, pence, pound
price
cost
buy
sell
spend, spent
pay
change
dear, costs more
cheap, costs less, cheaper
costs the same as
how much…? how many…?
total